1. Field of the Invention
The present invention relates to a sheet processing apparatus which carried out skew correction and punching processing to a sheet discharged from an image forming unit of MFP (multi-function peripheral), which is a digital multi-function machine, a copy machine, a printer or the like.
2. Description of the Related Art
In an image forming apparatus such as MFP, copy machine or printer, a post-processing device (finisher) is provided next to a paper discharge unit in the image forming apparatus body in order to carry out post-processing such as punching processing and staple processing to a sheet on which an image has been formed.
In such a post-processing apparatus, a sheet discharged from the image forming apparatus body may become slant (hereinafter referred to skew) with respect to the conveying direction. If punching processing (hole punching) is carried out to the skewed sheet, the hole punching position is deviated, causing a trouble at the time of filing. Therefore, a skew correcting unit is provided to correct the skew of the sheet and then punching processing is carried out.
JP-A-2000-153953 discloses a sheet processing apparatus in which a punching unit is movable in a direction that intersects the sheet conveying direction. In this example, the punching unit is moved from the home position (HP) into the direction that intersects the sheet conveying direction and carries out punching. During the operation to move the punching unit to HP after punching is finished, the punching unit is moved to the standby position.
JP-A-2006-16129 discloses a sheet processing apparatus having a pair of rollers for skew correction and a hole punching unit. In this example, the pair of roller for skew correction carries a sheet, and plural edge detection sensors are provided in order to detect the lateral edge of the conveyed sheet.
JP-A-10-194557 discloses a sheet hole punching apparatus having a detection unit which detects the lateral edge of a conveyed sheet. In this example, a hole punching unit is made movable in a direction orthogonal to the sheet conveying direction, and the moving position of the hole punching unit is decided in accordance with the result of detection by the detection unit.
JP-A-2005-31877 discloses a control apparatus for a motor used for conveying a sheet or the like. In this example, the apparatus has a first control system which moves a sheet at a constant speed up to a halfway position before reaching a target stop position, and a second control system which moves the sheet at a low speed from the halfway position to the target stop position. A motor is rotationally driven in two stages.
Moreover, JP-A-9-249348 discloses a punching processing apparatus in which a punching mechanism is movable in a direction orthogonal to the sheet conveying direction. In this example, prior to punching processing, the punching mechanism is moved to a predetermined standby position and caused to wait there. The standby position is preset according to the sheet size.
Meanwhile, high-speed processing and power saving are required of the recent image forming apparatus. As the image forming apparatus operates at a higher speed, the sheet conveying speed becomes higher. Therefore, at the time of punching processing, it is difficult to stop a sheet at a regular position and the position of the punch hole may be deviated. Also, skew correction may take time and measures must be taken to deal with high-speed processing. Moreover, measures for power saving are necessary.